1. Field of the Invention
This invention generally relates to explorations for hydrocarbons involving investigations of regions of an earth formation that nay be penetrated by a borehole. More specifically, the invention deals with the problem of accurate digital representation of signals measured by sensors in the earth formation for subsequent processing. An example is discussed in some detail of highly localized borehole investigations employing the introduction and measuring of individual focused survey currents injected toward the wall of a borehole with a tool moved along the borehole.
2. Background of the Art
Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. In an electrical investigation of a borehole, current from an electrode is introduced in the formation from a tool inside the borehole. There are two modes of operation. In the first mode, the current at the measuring electrode is maintained constant and a voltage is measured. In the second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain constant the voltage measured at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated.
Techniques for investigating the earth formation with arrays of measuring electrodes have been discussed. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Patent No. 685727 to Mann et al., U.S. Pat. No. 4,468,623 to Gianzero, and U.S. Pat. No. 5,502,686 to Dory et al. The Baker patent teaches a plurality of electrodes, each of which is formed of buttons which are electrically joined by flexible wires with buttons and wires embedded in the surface of a collapsible tube. The Mann patent teaches an array of small electrode buttons either mounted on a tool or a pad and each of which introduces in sequence a separately measurable survey current for an electrical investigation of the earth formation. The electrode buttons are placed in a horizontal plane with circumferential spacings between electrodes, and a device for sequentially exciting and measuring a survey current from the electrodes is described. The Gianzero patent discloses a pad mounted device, each pad having a plurality of small measure electrodes from which individually measurable survey currents are injected toward the wall of the borehole. The measure electrodes are arranged in an array in which the measure electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole. U.S. Pat. No. 6,348,796 to Evans et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, discloses a pad mounted resistivity device in which the electrodes are non-overlapping. U.S. Pat. No. 6,714,014 to Evans et al. having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, discloses a pad mounted resistivity imaging device that uses capacitive coupling and may be used with oil based mud.
The problem of the two-electrode system are typical of many that are encountered in the evaluation of earth formations. In general terms, a plurality of sensors make measurements of a parameter of interest of the earth formation. The signals associated with each of the sensors may be characterized as the sum of a background signal and a differential signal. The differential signal varies from one sensor to another and is characteristic of local variations in the parameter of interest. The signals at the sensors are analog measurements. In situations where the background signal is much greater than the differential signal, it is desirable to improve the dynamic range of the difference signals when the output of the sensors is part of a digital data processing system. The present invention addresses this problem. It should be noted that the term “formation” as used in this document is intended to include a formation matrix, a formation fluid and a borehole fluid.